Abstract For disk galaxies spirals and irregulars, the inner circular-velocity gradient dRV0 inner steepness of the rotation curve correlates with the central surface brightness ∑*,0 with a slope of ~0.5. This implies that the central dynamical mass density scales almost linearly with the central baryonic density. Here I show that this empirical relation is consistent with a simple model where the central baryonic fraction ƒbar,0 is fixed to 1 no dark matter and the observed scatter is due to differences in the baryonic mass-to-light ratio Mbar - LR ranging from 1 to 3 in the R-band and in the characteristic thickness of the central stellar component Δz ranging from 100 to 500 pc. Models with lower baryonic fractions are possible, although they require some fine-tuning in the values of Mbar-LR and Δz. Regardless of the actual value of ƒbar,0, the fact that different types of galaxies do not show strong variations in ƒbar,0 is surprising, and may represent a challenge for models of galaxy formation in a Λ Cold Dark Matter ΛCDM cosmology. View Full-Text